Thermally operated circuit breakers



March 13, F R- w STRAFFORD ETAL THERMALLY OPERATED CIRCUIT BREAKERS Filed Jan. 13, 1955 /NVE/YTOPS 5TH F'F'O K D PRES T GE 4770QNEY Js/vrvoRrH HARRY THOMAS FREDEKI cK RIC HNOND United States Patent 2,738,396 THERMALLY OPERATED CIRCUIT BREAKERS Frederick Richmond Wentworth Strafford and Harry Thomas Prestige, Enfield, England, assignors to Belling & Lee Limited, Enfield, England Application January 13, 1953, Serial No. 331,014

Claims priority, application Great Britain January 17, 1952 Claims. (Cl. 200-122) This invention relates to thermally operated circuit breakers for use in the protection of electrical apparatus in the event of a fault developing which causes excessive current to flow. The present invention is directed primarily to the protection of domestic mains operated radio and television receivers, light duty electrical domestic apparatus, electronic apparatus or telecommunication apparatus, in which a fault in one component can cause irreparable damage to the whole receiver and possibly cause a serious fire.

The provision of fuses on radio and television receivers is almost standard but it is rarely possible for a fuse to be used whose rated fusing current is that of the receiver since the initial current taken on switching on is often very much greater than the operating current. In the case of receivers operated from an A. C. supply by means of a transformer the transient current possible on switching on the receiver is a number of times greater than the normal operating current. Thus the failure of a component in a receiver which causes only a slight increase in the operating current will almost certainly not rupture the fuse, but will merely reduce its life. It is well known to employ fuses having a delayed action characteristic in that the fusing current has to flow for some seconds before fusing takes place whilst for a heavyoverload of short duration of about second fusing will not take place. Such fuses are not capable of giving eflicient protection because they are not very sensitive to small increases in current. Further, when such a fuse has ruptured it must be replaced and there is always the possibility of a replacement not being available. If cartridge fuses are used the cost of replacement is not negligible.

The present invention contemplates the provision of a thermally operated circuit breaker of miniature construction which is inexpensive to manufacture and can be designed to withstand an overload of some 10 to 50 times the normal operating current for about 5 second and to open circuit within seconds on an overload of 300 to 500 per cent. of the normal current. Thermally operated circuit breakers are known where contacts spring open when a bimetallic strip is overheated by the controlled current but such arrangements are comparatively expensive to manufacture.

A thermally operated circuit breaker in accordance with the present invention includes a metallic clip, a spring or other loaded contact normally retained by the clip against the loading force to complete the controlled circuit, and thermally operated means to open said clip, said thermally operated means coming into operation on the passage of an overload current through the circuit breaker to release the contact and thus break the controlled circuit. It is preferred that the clip should include one or more bimetallic strips to cause opening thereof under the influence of excessive heat and that the controlled circuit should include a resistance heating coil wound around the clip to supply heat to the bimetallic strips in accordance with the current flowing. Also it is preferred that the contact member should be spring loaded to move quickly from the clip when actually released.

In order that the invention may be clearly understood one embodiment thereof will now be described with reference to the accompanying drawings, in which:

Figure 1 is a plan view of the circuit breaker in its normal position;

Figure 2 is a plan view as in Figure 1 the contacts being in the broken position;

Figure 3 is an elevation showing the spring loading of the movable contact;

Figure 4 is a cross section on the line 44 of Figure 1; and

Figure 5 ure 2.

The embodiment shown in the drawings is intended for permanent fixing in a radio or television receiver the various elements being mounted on a small base board 10 of a synthetic resin material, this board being provided with two fixing holes 11 so that it may be secured by bolts or the like to the chassis of the receiver. Centrally mounted on the base board is the clip 12 which comprises a long length of bimetallic strip folded back on itself at position 13 and cranked at position 14 leaving a pair of free ends 15 closely adjacent one to another and held in spaced relationship from the base board 10. The clip is secured to the base board by means of an eyelet 16 passing through the cranked portion 14 and securing it to the base board whilst at position 13 a small rightangle bend is formed to engage in a hole 17 in the base board to secure the clip 12 against rotational movement about the eyelet. The extreme ends of the strips 15 are each formed by pressing into a transverse groove 18 and a guide portion 19, the guide portions merely being the extreme ends of the strips bent outwardly to form together a guide of V-shape leading into a recess formed by the two oppositely disposed grooves 18. A piece of asbestos insulation 21 is wrapped around the strip 15 and over this insulation a coil 22 of resistance wire is wound, one end 23 of the coil being taken back inside the insulation and secured under eyelet 16 to make electrical contact with the strip whilst the other end 24 of the coil leads from the coil to a connecting tag 25 secured to the base board. The insulation 21 and wire 22 are loosely wound on to the strips 15 in order to allow outward movement. The moving contact 26 and its helical loading spring 27 are integrally formed from one piece of metallic spring wire, such for example as Phosphor bronze, and the spring is secured to the base board by clamping its lower end 29 under the second connecting tag 28, the extreme end being bent at right angles and passing into a hole in the base board as shown in Figure 3. The contact 26 is formed by the upper end of the spring 27 the extreme end of this contact being bent at a right angle and carrying a piece of coloured sleeving 30 which acts as an indicator of the position of the contact. The fixing of the spring 27 and moving contact 26 is such that the contact 26 occupies the position shown in Figure 2 when the spring 27 is untensioned. By arranging the spring 27 in the manner shown the contact 26 is raised above the level of the base board 11) and pivotal movement of the moving contact on its mounting spring 27 will take the contact into or out of the V-shape formed by the guide portions 19. Inward movement of the contact will cam the guide portions 19 apart and the contact will move into the recess formed by grooves 18. The contact 26 being a wire of circular section is engaged in the grooves 18 with a clipping action and the spring tension is such that it normally applies sufiicient force to contact 26 to cause it to move out of the clip.

is a cross section on the line 55 of Fig- In use the circuit breaker is secured in a radio or television receiver and it is connected in the circuit to be protected by means of the two tags 25 and 28. When the contact 26 i held by the clip 12 a circuit will exist between tags 25 and 28 passing in orderfrom tag 25 through the connecting wire 24-, the resistance wire coil 22, wire 23, the strips 15,;and contact 26 back to tag 2-8.

Normally when passing the maximum rated current, heat is developed in the heating coil 22 as a result of wattage dissipation and the temperature to which the strips are raised, is just sufiicient not to allow sufficient opening of the clip to release the spring arm. A small overload of the order of 200 to 300 per cent, will cause.

more heat to be developed in the coil, to raise further the temperature of the strips 15 so that they will bend further apart and unclip the contact 26 thereby breaking connection between the arm and the strips. The coloured sleeving at the end of the free arm is an indication of operation of the circuit breaker and is easily noted on inspection,

In order to make the circuit again it is only necessary to push the contact 26 to clip. it into position between the bimetal strips 15 and the device is thus reset and cape: ble again of breaking circuit under the same conditions of overload as before.

In another embodiment of the invention, not illustrated, mounted in a cartridge similar to the normal cartridge fuse casing the two bimetallic strips are shaped and secured together by welding for part of their length, the heating coil being wound over the welded part. The welded end of the strip is secured by an insulating strip to one metallic end cap of the cartridge, the heater coil being connected one end to the bimetal strips and the other end to the cap. At the opposite end of the cartridge a helical spring of Phosphor bronze wire is positioned coaxially with the cartridge, the end adjacent the end cap. being secured thereto such as by soldering. At the free end of the spring, a length of Wire integral with the spring is bent transversely of the spring axis to form a contact to be clipped in the end of the bimetal strips and from thisv contact the wire is bent back axially to extend alongv the centre of the spring and out through a hole in the end cap to which the spring is secured. The projectingendhas a piece of coloured sleeving secured thereto.

The operation of this device is similar to the operation of the previously described device when it is placed in a end, the free ends of said strips being formed to define movable contact grasping portions, a movable contact yieldably urged to a circuit breaking position remote from said strips and displaceable to an operative position engaged between said grasping portions of the strips to be there retained by said grasping portions, and means for heating said strips in accordance with the current passing between said strips and said movable contact, said strips when heated being arranged to spread apart at said free ends thereof to release said movable contact from said grasping portions in response to the heating of both said strips to a degree corresponding to the flow of excessive current between said strips and movable contact.

2. A thermally operated circuit breaker comprising a fixed contact including a pair of bimetallic strips arranged in face-to-face relationship and secured together at one end, the free ends of said strips being formed to define movable contact grasping portions, a movable contact yieldably urged to a circuit breaking position remote from said strips and displaceable to an operative position engaged between said grasping portions, and means for heating said strips in accordance with the current passing between said strips and said movable contact, said strips being arranged to spread apart at said free ends thereof to release said movable contact from said grasping portions in response to the heating of said strips to a degree corresponding to the flow of excessive current between said strips and movable contact, said grasping portions of the bimetallic strips including transverse grooves opening toward each other and disposed adjacent to, and spaced by a small distance from, said free ends of the strips, the parts of said strips between said grooves and said free ends diverging toward the latter to define guide portions for leading the movable contact into said grooves.

3. A thermally operated circuit breaker according to claim 2; wherein said movable contact consists of spring metal wire of circular cross-section manually movable past said guide portions and into said grooves to be retained in the latter.

4. A thermally operated circuit breaker comprising a fixed contact including a pair of bimetallic strips arranged in face-to-face relationship and secured together at one end, the free ends of said strips being formed to define cartridge fuse holder the two end caps making connection with the spring grips. This device can be used toreplace a conventional cartridge fuse of equal rated current.

.Whilst circuit breakers in accordance with the inventionare fairly sensitive it is, preferable in radio and television not to place reliance on one breaker in the mains supply circuit only but to include breakers in the various subsidiary circuits of the receiver.

As a very simple example. applicable to a mains opcrated domestic radio receiver, one breaker is included in the mains supply circuit, a second breaker in series with the rectifier supplying the reservoir capacitor of the high tension smoothing circuit and a third breaker in series with the feed from the smoothing choke to the second smoothing capacitor. Thus the second and third breakers, each protect against serious leakage or complete breakdown in its associated capacitor and also against increases in high tension currents over the normal values whilst the, first breaker will protect mainly against valve heater short circuits and transformer faults.

It will be appreciated that the construction in accord-- ancewith the invention is extremely inexpensive to manufacture and, when installed for use may be reset after" breaking circuit involving no further expense and still giving the same protection.

We claim:

1'. A thermally operated. circuit breaker comprising a fixed contact including a pair of bimetallic strips arranged 1n face-to-face relationship and secured together at one movable contact grasping portions, a movable contact yieldably urged to a circuit breaking position remote from said strips and displaceable to an operative position engaged between said grasping portions of the strips to be there retained by said grasping portions, and means for heating said strips in accordance with the current passing between said strips and said movable contact, said strips being arranged to spread apart at said free ends thereof to release said movable contact from said grasping portions in response to the heating. of said strips to a degree corresponding to the flow of excessive current between said strips and movable contact, said movable contact consisting of a length of spring metal wire fixedly mounted at one end and formed with a resilient coil intermediate its ends to yieldably urge the other end of said wire out from between said grasping portions of the bimetallic strips.

5. A thermally operated circuit breaker according to claim 4; further comprising a colored sleeve mounted on said free end of the; movable contact to indicate, by its position, the condition of the circuit through the circuit breaker.

6. A thermally operated circuit breaker comprising a fixed contact including a pair of bimetallic strips arranged in face-to-face relationship and secured together at one end, the free ends of said strips. being formed. to.- define movable. contact grasping portions, a movable contact yieldably urgedto a. circuit breaking position remotefrom said strips, and. displaceable to an operative position engaged between said. grasping; portions of the, strips to be there retained by said grasping portions, and: means" for heating said strips in accordance with the current passing between said strips and said movable contact, said strips being arranged to spread apart at said free ends thereof to release said movable contact from said grasping portions in response to the heating of said strips to a degree corresponding to the flow of excessive current between said. strips and movable contact, said means for heating the bimetallic strips including a coil of high resistance wire connected in series with said strips and disposed in close proximity to the latter.

7. A thermally operated circuit breaker comprising a flat base of insulating material, a fixed contact including a pair of bimetallic strips disposed in face-to-face relationship and extending substantially parallel to said base, means securing together said strips at one end and there mounting said strips on said base so that the free ends of said strips can move toward and away from each other, a movable contact formed of a length of spring metal wire having a resilient coil intermediate its ends and with the opposite end portions extending tangentially from said coil means securing one of said end portions of the movable contact on said base so that the other end portion of the spring metal wire is movable between a torsionally urged circuit breaking position remote from said free ends of the bimetallic strips and an operative position engaged between said free ends of the strips, said free ends of the strips being formed to define grasping portions for retaining said other end portion of the spring metal wire therebetween against the force of said coil when said strips are disposed relatively close together, said strips being arranged to spread apart at said free ends for releasing said other end portion of the spring metal wire when said strips are heated beyond a predetermined degree, a first terminal on said base connected to said one end portion of the movable contact, an electrical insulating sleeve loosely disposed around said strips to permit the relative movements of the latter, an electrical resistance wire wound around said sleeve and connected at one end to said strips, and a second terminal on said base connected to the other end of said resistance wire so that, when said terminals are connected in a circuit the current flowing through said resistance wire, strips and movable contact wire heats the resistance wire to cause spreading apart of said grasping portions of the strips if the current becomes excessive.

8. A thermally operated circuit breaker according to claim 7; wherein said strips are otf-set at a location adjacent said one end secured to the base so that the major portions of said strips are spaced from the base, and said coil of the movable contact wire is disposed with its axis normal to said base so that said other end portion thereof is movable in a plane substantially parallel to said base between said circuit breaking and operative positions.

9. A thermally operated circuit breaker according to claim 7; wherein said two bimetallic strips are integrally joined together at said one end and are formed of a single long strip folded upon itself at the center.

10. A thermally operated circuit breaker according to claim 7; wherein said grasping portions of the bimetallic strips are defined by lateral grooves formed in said strips adjacent and slightly spaced from said free ends of the latter and opening toward each other, and the parts of the strips between said grooves and said free ends diverge toward the latter to guide the movable contact into engagement with said grooves.

References Cited in the file of this patent UNITED STATES PATENTS 928,402 Richardson et al. July 20, 1909 1,373,128 Freese Mar. 29, 1921 1,527,645 Hall Feb. 24, 1925 2,147,754 Rivers Feb. 21, 1939 2,271,729 Baldwin et al. Feb. 3, 1942 2,352,713 Hodgkins July 4, 1944 2,391,680 Cook Dec. 25, 1945 

